受污染地下水之複合式生物脫硝系統開發

Abstract

利用生物異營脫硝技術去除地下水硝酸鹽雖然是一種經濟有效的方法，但處理水中含有殘餘有機化合物、消毒副產物和微生物，對於飲用水仍有潛在風險。本研究結合多孔性擔體及薄膜生物處理技術，先利用生物槽中之多孔性擔體進行生物異營脫硝處理，而多餘碳源、消毒副產物前驅物和微生物，再利用薄膜生物反應槽（MBR）或生物擔體反應槽（BCR）進行分解及過濾去除。本研究藉由改變生物脫硝程序之碳氮比及體積負荷，探討去除硝酸鹽、亞硝酸鹽、溶解性有機碳（DOC）、溶解性微生物產物（SMPs）、總菌落數、大腸桿菌、濁度及粒徑分佈之效能影響。而微生物之行為表現，如脫硝菌活性、菌相、菌種鑑定及微生物增殖也都被詳細檢討。此外比較不同MBR薄膜之過濾阻力及處理水消毒後副產物之差異；最後篩選出BCR結合MBR之最適組合，設立模型設備於淨水場，進行長期試驗，評估應用之可行性。 研究結果顯示若控制較高碳氮比，其處理水硝酸鹽濃度較低，但DOC濃度較高；當提昇硝酸鹽體積負荷時，則處理水硝酸鹽濃度上升。BCR去除DOC之效能，明顯比MBR差；而當硝酸鹽氮體積負荷介於0.25 ~ 0.5 kg/m3 d之間，MBR去除SMP之效能比BCR為佳，但在硝酸鹽氮體積負荷0.75 kg/m3 d時，兩者去除SMP之效能則相近；MBR處理水之總菌落數及大腸桿菌水質指標皆優於BCR，而MBR處理水大腸桿菌及加氯消毒後之總菌落數皆可符合飲用水管制標準；去除濁度與過濾介質及孔徑相關，優劣順序為MBR-G(UF)、MBR-K(MF)及BCR(多孔性擔體) ；MBR-K過濾總阻力小於MBR-G，而MBR過濾總阻力也會隨MLSS濃度下降而降低；脫硝菌經過馴養後，最大氮氣產氣量增加4倍；採用nirS基因之引子將微生物進行DNA萃取及聚合酶連鎖反應(PCR)，顯示使用nirS引子可快速檢測處理槽中脫硝菌之菌群；應用螢光染色技術於好氧生物系統，顯示MBR及BCR微生物活性具佳，而使用變性梯度凝膠電泳(DGGE)，可發現BCR-N、MBR及BCR-C有不同之優勢菌種；添加次氯酸鈉在處理水中進行消毒試驗，其三鹵甲烷濃度低於80 μg/L符合飲用水之法規規定。 以薄膜生物脫硝技術進行淨水場模型廠試驗，硬體設置於已停用淨水場，其硝酸鹽氮平均濃度約40 mg/L，出水量達18 ~ 36 CMD，造水率達99.5%，無氧槽之脫硝效率達85% 以上。新穎性複合式脫硝除碳系統，當控制碳氮比為1.5及硝酸鹽氮體積負荷低於0.8 kg NO3-N / m3 d時，能夠有效去除硝酸鹽及亞硝酸鹽，且薄膜出水之DOC濃度及SMPs濃度明顯比脫硝槽低，證實長期操作穩定性佳。 綜上所述，本研究開發處理受污染地下水之複合式生物脫硝系統，藉由實驗室試驗及模型廠驗證，證實可有效去除硝酸鹽、亞硝酸鹽及分解多餘碳源，降低消毒副產物之前驅物及微生物含量，進而提昇飲用水安全。未來可應用於硝酸鹽含量高之表面水及地下水處理，亦可應用於含高硝酸鹽之工業廢水處理。

Heterotrophic denitrification is an economical means for removing nitrate from groundwater. However, residual organic compounds, disinfection by-products and microorganism contamination in treated water cause potential risks to drinking water supplies. To overcome these problems, system combines with Bio-Carrier Reactor (BCR) for heterotrophic denitrification and Membrane Bioreactor (MBR) for removal of residual organic compounds, precursor of disinfection by-products and microorganisms was studied. The effect on nitrate, nitrite, dissolved organic carbon (DOC), Soluble microbial products (SMPs), total bacteria count, coliforms, turbidity and particle size distribution were investigated by change of carbon to nitrogen ratio and volumetric loading in denitrification process. The performance of microorganisms, like denitrification bacteria activity, microbial community identification and microorganism growth were also studied. Filtration resistance and disinfection by-products were compared for different membranes used in MBR reactor. Finally, combination of BCR and MBR was chosen as a pilot plant for a long-term experiment in field to evaluate the feasibility of application. The research results showed that the nitrate concentration in effluent was lower by controlling higher C/N ratio; however, the residual DOC concentration was higher too. The nitrate concentration in the MBR effluent was increased when the nitrate volumetric loading in the denitrification tank was increased. MBR showed better performance in DOC removal compared to BCR. Also MBR showed better removal of SMP when nitrate volumetric loading was in the range of 0.25 to 0.5 kg NO3--N/m3 d. However, the removal of SMP showed no significant difference between MBR and BCR when nitrate volumetric loading was at 0.75 kg NO3--N/m3 d. Total bacteria count and coliforms in MBR effluent was lower than those in BCR. Also, Coliforms in MBR effluent and total bacteria count after chlorine addition could comply with drinking water quality standard. The removal of turbidity was related with filter media and its pore size. The performance was decreased in the order of MBR-G(UF), MBR-K(MF) and BCR(porous carrier). The total filtration resistance for MBR-K was lower than that for MBR-G. And, the total filtration resistance for MBR was decreased as the MLSS concentration was decreased. The maximum nitrogen gas production after 24 hours increased four times after acclimation of denitrification bacteria. Using primer of nirS gene in extraction of Deoxyribonucleic acid (DNA) from microorganism and Polymerase Chain Reaction (PCR) showed that primer of nirS gene could detect denitrification bacteria in reactor rapidly. Application of fluorescence dyeing technology to aerobic biological system showed that MBR microorganism had similar activity compared to BCR. Denaturing Gradient Gel Electrophoresis (DGGE) analysis found that BCR-N, MBR and BCR-C had different dominant microorganisms. Trihalomethanes concentration in the effluent was below 80 μg/L of drinking water standard after addition of sodium hypochlorite for disinfection. A pilot plant combining BCR and MBR was installed in the field for long-term test. The groundwater used as influent containing 40 mg NO3--N/L nitrate in average. During the long-term test, the treated water was controlled at 18 to 36 CMD, the water productivity was at 99.5% and the denitrification efficiency in anoxic tank was 85%. It shows that the effluent nitrate and nitrite were both removed effectively by controlling carbon to nitrogen ratio at 1.5 (g/g) and volumetric loading of nitrate nitroge below 0.8 kg NO3-N/m3d in the influent. It also showed that amounts of DOC and SMPs in membrane permeate are significantly lower than those in the treated water of denitrification reactor. Therefore, a hybrid biological system was proposed in this study. Bench and pilot experiment results showed it could remove nitrate, nitrite, residual organic compounds, precursors of disinfection by-products and microbial products effectively to improve drinking water safety. Based on the research results, the process could be applied in the treatment of surface water and groundwater for nitrate removal. It was also highly potential for treatment of industrial wastewater with high nitrate concentration.